There is also a small amount of vitamin C(ascorbic acid) present in raw milk but is very heat-labile and easily destroyed by pasteurization Minerals All 22 minerals considered to be essential to the human diet are present in milk. These include three families of salts 1. Sodium(Na), Potassium(K)and Chloride( CD: These free ions are negatively correlated to lactose to maintain osmotic equilibrium of milk with blood Calcium (Ca), Magnesium Mg), Inorganic Phosphorous (P(i), and Citrate This group consists of 2/3 of the Ca, 1/3 of the Mg, 1/2 of the P(, and less than 1/10 of the citrate in colloidal(nondiffusible) form and present in the casein micelle 3. Diffusible salts of Ca, Mg, citrate, and phosphate: These salts are very ph dependent and contribute to the overall acid- base equilibrium of milk The mineral content of fresh milk is given below Mineral Content per litre Sodium (mg) 350-900 1100-1700 Chloride(mg) 900-1100 Calcium(mg) 1100-1300 Magnesium(mg) 90-140 Phosphorus(mg) 900-1000 Iron(ug) 300-60 Zinc(ug) 2000-6000 Copper (ug) 100-600 Manganese(ug) 260 Fluoride(ug) 30-220 Selenium(ug) 5-67 Cobalt 0.5-1.3 Chromium (ug 8-13 Molybdenum(ug) 18-120 Nickel (ug) Silicon(ug) 750-7000 Vanadium(ug) tr-310 40-500 Arsenic (ug) 20-60 Physical Properties Density
26 There is also a small amount of vitamin C (ascorbic acid) present in raw milk but is very heat-labile and easily destroyed by pasteurization. Minerals All 22 minerals considered to be essential to the human diet are present in milk. These include three families of salts: 1. Sodium (Na), Potassium (K) and Chloride (Cl):These free ions are negatively correlated to lactose to maintain osmotic equilibrium of milk with blood. 2. Calcium (Ca), Magnesium (Mg), Inorganic Phosphorous (P(i)), and Citrate: This group consists of 2/3 of the Ca, 1/3 of the Mg, 1/2 of the P(i), and less than 1/10 of the citrate in colloidal (nondiffusible) form and present in the casein micelle. 3. Diffusible salts of Ca, Mg, citrate, and phosphate: These salts are very pH dependent and contribute to the overall acid-base equilibrium of milk. The mineral content of fresh milk is given below: Mineral Content per litre --------------------------------------------------------------------------------- Sodium (mg) 350-900 Potassium (mg) 1100-1700 Chloride (mg) 900-1100 Calcium (mg) 1100-1300 Magnesium (mg) 90-140 Phosphorus (mg) 900-1000 Iron (ug) 300-600 Zinc (ug) 2000-6000 Copper (ug) 100-600 Manganese (ug) 20-50 Iodine (ug) 260 Fluoride (ug) 30-220 Selenium (ug) 5-67 Cobalt (ug) 0.5-1.3 Chromium (ug) 8-13 Molybdenum (ug) 18-120 Nickel (ug) 0-50 Silicon (ug) 750-7000 Vanadium (ug) tr-310 Tin (ug) 40-500 Arsenic (ug) 20-60 Physical Properties Density
The density of milk and milk products is used for the following: o to convert volume into mass and vice versa o to estimate the solids content o to calculate other physical properties(e.g. kinematic viscosity) Density, the mass of a certain quantity of material divided by its volume, is dependant on the following o temperature at the time of measurement o temperature history of the material o composition of the material (especially the fat content o inclusion of air(a complication with more viscous products) With all of this in mind, the density of milk varies within the range of 1027 to 1033 kgm(-3)at20°C The following table gives the density of various fluid dairy products as a function of fat and solids-not-fat(SNF)composition Product Density(kg/L)at Composition Product Fat(%)SNF(%)44°C10°C20°C38.9。C Producer milk 4.008.95 1.03510331.0301023 Homogenized milk 3.6 8.6 1.0331.03210291022 Skim milk, pkg 0.028.9 10361.03510331.026 Fortified skim 0.0210.151.0411.041.0381.031 Half and half 12.257.75 1.0271.0251.0201.010 Half and half. fort. 11.3 8.9 10311.03010241.014 Light cream 20 72 1.0211.0181.0121000 Heavy cream 5.55 1.00810050.9940.978 Viscosity iscosity of milk and milk products is important in determining the following o the rate of creaming o rates of mass and heat transfer o the flow cond itions in dairy processes Milk and skim milk, excepting cooled raw milk, exhibit Newtonian behavior, in
27 The density of milk and milk products is used for the following; o to convert volume into mass and vice versa o to estimate the solids content o to calculate other physical properties (e.g. kinematic viscosity) Density, the mass of a certain quantity of material divided by its volume, is dependant on the following: o temperature at the time of measurement o temperature history of the material o composition of the material (especially the fat content) o inclusion of air (a complication with more viscous products) With all of this in mind, the density of milk varies within the range of 1027 to 1033 kg m(-3) at 20° C. The following table gives the density of various fluid dairy products as a function of fat and solids-not-fat (SNF) composition: Product Density (kg/L) at: Composition __________________________________________________________________ Product Fat (%) SNF (%) 4.4° C 10° C 20°C 38.9° C ____________________________________________________________________ Producer milk 4.00 8.95 1.035 1.033 1.030 1.023 Homogenized milk 3.6 8.6 1.033 1.032 1.029 1.022 Skim milk, pkg 0.02 8.9 1.036 1.035 1.033 1.026 Fortified skim 0.02 10.15 1.041 1.04 1.038 1.031 Half and half 12.25 7.75 1.027 1.025 1.020 1.010 Half and half, fort. 11.3 8.9 1.031 1.030 1.024 1.014 Light cream 20.00 7.2 1.021 1.018 1.012 1.000 Heavy cream 36.60 5.55 1.008 1.005 0.994 0.978 ____________________________________________________________________ Viscosity Viscosity of milk and milk products is important in determining the following: o the rate of creaming o rates of mass and heat transfer o the flow conditions in dairy processes Milk and skim milk, excepting cooled raw milk, exhibit Newtonian behavior, in
which the viscosity is independent of the rate of shear. The viscosity of these products depends on the following cooler temperatures increase viscosity due to the increased voluminosity of casein micelles temperatures above 65C increase viscosity due to the denaturation of whey proteins pH: an increase or decrease in pH of milk also causes an increase in casein micelle voluminosity Cooled raw milk and cream exhibit non-Newtonian behavior in which the viscosity is dependant on the shear rate. Agitation may cause partial coalescence of the fat globules(partial churning) which increases viscocity Fat globules that have under gone cold agglutination, may be dispersed due to agitation, causing a decrease in Freezing point Freezing point is a colligative property which is determined by the molarity of solutes rather than by the percentage by weight or volume. In the dairy industry, freezing point is mainly used to determine added water but it can also been used to powder,and to determine water activity of cheese. The freezing point of< determine lactose content in milk, estimate whey powder contents in skim mi usually in the range of-0 512 to-0.550%C with an average of about -0 522C Correct interpretation of freezing point data with respect to added water depends on a good understand ing of the factors affecting freezing point depression. With respect to interpretation of freezing points for added water determination, the most significant variables are the nutritional status of the herd and the access to water Under feeding causes increased freezing points. Large temporary increases in freezing point occur after consumption of large amounts of water because milk is iso-osmotic with blood. The primary sources of non-intentional added water in milk re residual rinse water and condensation in the milking system Acid-Base Equilibri Both titratable acidity and ph are used to measure milk acidity. The pH of milk at 25C normally varies within a relatively narrow range of 6.5 to 6.7. The normal range for titratable acid ity of herd milks is 13 to 20 mmolL. Because of the large inherent variation, the measure of titratable ac id ity has little practical value except to measure changes in ac id ity (eg, during lactic fermentation) and even for this purpose, pH is a better measurement There are many components in milk which prov ide a buffering action. The major buffering groups of milk are caseins and phosphate
28 which the viscosity is independent of the rate of shear. The viscosity of these products depends on the following: o Temperature: ▪ cooler temperatures increase viscosity due to the increased voluminosity of casein micelles ▪ temperatures above 65° C increase viscosity due to the denaturation of whey proteins o pH: an increase or decrease in pH of milk also causes an increase in casein micelle voluminosity Cooled raw milk and cream exhibit non-Newtonian behavior in which the viscosity is dependant on the shear rate. Agitation may cause partial coalescence of the fat globules (partial churning) which increases viscocity. Fat globules that have under gone cold agglutination, may be dispersed due to agitation, causing a decrease in viscosity. Freezing Point Freezing point is a colligative property which is determined by the molarity of solutes rather than by the percentage by weight or volume. In the dairy industry, freezing point is mainly used to determine added water but it can also been used to determine lactose content in milk, estimate whey powder contents in skim milk powder, and to determine water activity of cheese. The freezing point of milk is usually in the range of -0.512 to -0.550° C with an average of about -0.522° C. Correct interpretation of freezing point data with respect to added water depends on a good understanding of the factors affecting freezing point depression. With respect to interpretation of freezing points for added water determination, the most significant variables are the nutritional status of the herd and the access to water. Under feeding causes increased freezing points. Large temporary increases in freezing point occur after consumption of large amounts of water because milk is iso-osmotic with blood. The primary sources of non-intentional added water in milk are residual rinse water and condensation in the milking system. Acid-Base Equilibria Both titratable acidity and pH are used to measure milk acidity. The pH of milk at 25° C normally varies within a relatively narrow range of 6.5 to 6.7. The normal range for titratable acidity of herd milks is 13 to 20 mmol/L. Because of the large inherent variation, the measure of titratable acidity has little practical value except to measure changes in acidity (eg., during lactic fermentation) and even for this purpose, pH is a better measurement. There are many components in milk which provide a buffering action. The major buffering groups of milk are caseins and phosphate
Optical Properties Optical properties prov ide the basis for many rapid, indirect methods of analysis such as proximate analysis by infrared absorbency or light scattering. Optical properties also determine the appearance of milk and milk products. Light scattering by fat globules and case in micelles causes milk to appear turbid and opaque. Light scattering occurs when the wave length of light is near the same magnitude as the particle. Thus, smaller particles scatter light of shorter wavelengths. Skim milk appears slightly blue because casein micelles scatter the shorter wavelengths of visible light(blue) more than the red. The carotenoid precursor of vitamin A, B -carotene, contained in milk fat, is responsible for the 'creamy' colour of milk Riboflavin imparts a greenish colour to whey Refractive index (ri is normally determined at 20C with the d line of the sodium spectrum. The refractive index of milk is 1. 3440 to 1.3485 and can be used to estimate total solids ChAPTER 4 Dairy microbiology Basic Microbiology Microorganisms Microorganisms are living organ isms that are individually too small to see with the naked eye. The unit of measurement used for microorganisms is the micrometer (u m); 1 um 0.001 millimeter; I nanometer(nm)=0.001 u m. Microorganisms are found everywhere (ubiquitous)and are essential to many of our planets life processes. With regards to the food industry, they can cause spoilage, prevent spoilage through fermentation, or can be the cause of human illness There are several classes of microorganisms, of which bacteria and fungi (yeasts and moulds) will be discussed in some detail. Another type of microorganism, the bacterial viruses or bacteriophage, will be examined in a later section Bacteria Bacteria are relatively simple single-celled organisms. One method of classification is by shape or morphology Cocci: spherical shape 04-1.5μ Examples: staphylo cocci- form grape-like clusters; streptococci-form bead-like
29 Optical Properties Optical properties provide the basis for many rapid, indirect methods of analysis such as proximate analysis by infrared absorbency or light scattering. Optical properties also determine the appearance of milk and milk products. Light scattering by fat globules and casein micelles causes milk to appear turbid and opaque. Light scattering occurs when the wave length of light is near the same magnitude as the particle. Thus, smaller particles scatter light of shorter wavelengths. Skim milk appears slightly blue because casein micelles scatter the shorter wavelengths of visible light (blue) more than the red. The carotenoid precursor of vitamin A, ß -carotene, contained in milk fat, is responsible for the 'creamy' colour of milk. Riboflavin imparts a greenish colour to whey. Refractive index (RI) is normally determined at 20° C with the D line of the sodium spectrum. The refractive index of milk is 1.3440 to 1.3485 and can be used to estimate total solids. CHAPTER 4 Dairy Microbiology Basic Microbiology Microorganisms Microorganisms are living organisms that are individually too small to see with the naked eye. The unit of measurement used for microorganisms is the micrometer (µ m); 1 µ m = 0.001 millimeter; 1 nanometer (nm) = 0.001 µ m. Microorganisms are found everywhere (ubiquitous) and are essential to many of our planets life processes. With regards to the food industry, they can cause spoilage, prevent spoilage through fermentation, or can be the cau se of human illness. There are several classes of microorganisms, of which bacteria and fungi(yeasts and moulds) will be discussed in some detail. Another type of microorganism, the bacterial viruses or bacteriophage, will be examined in a later section. Bacteria Bacteria are relatively simple single-celled organisms. One method of classification is by shape or morphology: • Cocci: - spherical shape - 0.4 - 1.5 µ m Examples: staphylococci - form grape-like clusters; streptococci - form bead-like chains
Rods 0. 25-1.0 u m width by 0.5-6.0 u m long Examples: bacilli-straight rod; spirilla-spiral rod There exists a bacterial system of taxonomy, or class ification system, that is internationally recognized with family, genera and species divisions based on genetics Some bacteria have the ability to form resting cells known as endospores. The spore forms in times of environmental stress, such as lack of nutrients and moisture needed for growth, and thus is a survival strategy. Spores have no metabolism and can withstand ad verse conditions such as heat, disinfectants, and ultraviolet light. When the environment becomes favourable, the spore germinates and giving rise to a single vegetative bacterial cell. Some examples of spore-formers important to the food industry are members of Bacillus and Clostridium generas Bacteria reproduce asexually by fission or simple division of the cell and its contents. The doubling time, or generation time, can be as short as 20-20 min. Since each cell grows and divides at the same rate as the parent cell, this could under favourable cond itions translate to an increase from one to 10 million cells in 1l hours! However, bacterial growth in reality is limited by lack of nutrients, accumulation of toxins and metabolic wastes, unfavourable temperatures and dessication. The maximum number of bacteria is approximately 1 X 10e9 CFU/g or ml Note: Bacterial populations are expressed as colony forming units(CfU) per gram or millilitre Bacterial growth generally proceeds through a series of phases Lag phase: time for microorganisms to become accustomed to their new environment. There is little or no growth during this phase Log phase: bacteria logarithmic, or exponential, growth begins; the rate of multiplication is the most rapid and constant Stationary phase: the rate of multiplication slows down due to lack of nutrients and build-up of toxins. At the same time, bacteria are constantly dy ing so the numbers actually remain constant Death phase: cell numbers decrease as growth stops and existing cells die off. The shape of the curve varies with temperature, nutrient supply, and other growth factors This exponential death curve is also used in modeling the heating destruction of microorganisms Yeasts
30 • Rods: - 0.25 - 1.0 µ m width by 0.5 - 6.0 µ m long Examples: bacilli - straight rod; spirilla - spiral rod There exists a bacterial system of taxonomy, or classification system, that is internationally recognized with family, genera and species divisions based on genetics. Some bacteria have the ability to form resting cells known as endospores. The spore forms in times of environmental stress, such as lack of nutrients and moisture needed for growth, and thus is a survival strategy. Spores have no metabolism and can withstand adverse conditions such as heat, disinfectants, and ultraviolet light. When the environment becomes favourable, the spore germinates and giving rise to a single vegetative bacterial cell. Some examples of spore-formers important to the food industry are members of Bacillus and Clostridium generas. Bacteria reproduce asexually by fission or simple division of the cell and its contents. The doubling time, or generation time, can be as short as 20-20 min. Since each cell grows and divides at the same rate as the parent cell, this could under favourable conditions translate to an increase from one to 10 million cells in 11 hours! However, bacterial growth in reality is limited by lack of nutrients, accumulation of toxins and metabolic wastes, unfavourable temperatures and dessication. The maximum number of bacteria is approximately 1 X 10e9 CFU/g or ml. Note: Bacterial populations are expressed as colony forming units (CFU) per gram or millilitre. Bacterial growth generally proceeds through a series of phases: • Lag phase: time for microorganisms to become accustomed to their new environment. There is little or no growth during this phase. • Log phase: bacteria logarithmic, or exponential, growth begins; the rate of multiplication is the most rapid and constant. • Stationary phase: the rate of multiplication slows down due to lack of nutrients and build-up of toxins. At the same time, bacteria are constantly dying so the numbers actually remain constant. • Death phase: cell numbers decrease as growth stops and existing cells die off. The shape of the curve varies with temperature, nutrient supply, and other growth factors. This exponential death curve is also used in modeling the heating destruction of microorganisms. Yeasts